The etiologies of dental caries and periodontal diseases are closely associated with dental plaque, the biofilm that develops from microbial colonization of the tooth surface. Colonization is initiated by a limited number of gram positive species, primarily viridans group streptococci and actinomyces. These bacteria typically occur in vivo as members of a complex microbial community, the formation of which depends on an array of specific adhesive interactions. Interactions between different bacteria, detected by in vitro coaggregation, are thought to play an important role in biofilm development. Coaggregations between actinomyces and streptococci or between different streptococci generally depend on GalNAc- or Gal-sensitive adhesins present on the actinomycete or on certain streptococci and complementary receptor molecules present on other strains of at least four viridans group streptococcal species. The adhesins of actinomyces are associated with the type 2 fimbriae of these bacteria and the receptors of streptococci with a family of structurally related cell wall polysaccharides. Type 2 fimbriae interact with either of two host-like disaccharide motifs that occur within the oligosaccharide repeating units of different streptococcal receptor polysaccharides. These host-like motifs are relatively non-immunogenic and consequently, the antigenicity of receptor polysaccharides is more closely correlated with other features of the polysaccharide chain. Current studies are directed toward defining the genetic basis of receptor polysaccharide structure and function. Genes for polysaccharide biosynthesis have been identified at four distinct loci of the streptococcal chromosome. Information gained from these studies should contribute to the development of molecular strategies to further assess the recognition role of streptococcal receptor polysaccharides in oral biofilm formation and hopefully, improved approaches for the prevention of plaque-related diseases.